The spinal column is a highly complex system of bones and connective tissues that provide support for the body and protect the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked atop one another, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord is located behind the vertebral bodies. The spine has a natural curvature (i.e., lordosis in the lumbar and cervical regions and kyphosis in the thoracic region) such that the endplates of the upper and lower vertebrae are inclined towards one another.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), excess kyphosis (abnormal forward curvature of the spine), excess lordosis (abnormal backward curvature of the spine), spondylolisthesis (forward displacement of one vertebra over another), and other disorders caused by abnormalities, disease, or trauma (such as ruptured or slipped discs, degenerative disc disease, fractured vertebrae, and the like). Patients that suffer from such conditions often experience extreme and debilitating pain, as well as diminished nerve function. Posterior fixation for spinal fusions, decompression, deformity, and other reconstructions are performed to treat these patients. The aim of posterior fixation in lumbar, thoracic, and cervical procedures is to stabilize the spinal segments, correct multi-axis alignment, and aid in optimizing the long-term health of the spinal cord and nerves.
Spinal deformity is the result of structural change to the normal alignment of the spine and is usually due to at least one unstable motion segment. The definition and scope of spinal deformity, as well as treatment options, continues to evolve. Surgical objectives for spinal deformity correction include curvature correction, prevention of further deformity, improvement or preservation of neurological function, and the restoration of sagittal and coronal balance. Sagittal plane alignment and parameters in cases of adult spinal deformity (ASD) are becoming increasingly recognized as correlative to health related quality of life score (HRQOL). In the literature, there are significant correlations between HRQOL scores and radiographic parameters such as Sagittal Vertical Axis (SVA), Pelvic Tilt (PT) and mismatch between pelvic incidence and lumbar lordosis.
The SRS-Schwab classification of ASD was developed to assist surgeons with a way to categorize ASD, and provide methods of radiographic analysis. This classification system helps provide a protocol for pre-operative treatment planning and post-op assessment. The current environment to utilize this classification system requires surgeons to examine pre-operative patient films and measure pelvic incidence, lumbar lordosis, pelvic tilt, and sagittal vertical axis either manually or through the use of pre-operative software. After the procedure, the surgeon examines the post-operative films and measures the same parameters and how they changed as a result of the surgery. A need exists for systems and methods for assessing these and other spinal parameters intraoperatively and assessing changes to these intraoperative spinal parameters as a surgical procedure progresses towards a pre-operative plan.
During spinal surgeries, screws, hooks, and rods are devices used to stabilize the spine. Such procedures often require the instrumentation of many bony elements. The devices, for example rods, can be extremely challenging to design and implant into the patient. Spinal rods are usually formed of stainless steel, titanium, cobalt chrome, or other similarly hard metal, and as such are difficult to bend without some sort of leverage-based bender. Moreover, a spinal rod needs to be oriented in six degrees of freedom to compensate for the anatomical structure of a patient's spine as well as the attachment points (screws, hooks, etc.) for securing the rod to the vertebrae. Additionally, the physiological problem being treated as well as the physician's preferences will determine the exact configuration necessary. Accordingly, the size, length, and particular bends of the spinal rod depends on the size, number, and position of each vertebrae to be constrained, the spatial relationship amongst vertebrae, as well as the screws and hooks used to hold the rods attached to the vertebrae.
The bending of a spinal rod can be accomplished by a number of methods. The most widely used method is a three-point bender called a French Bender. The French bender is a pliers-like device that is manually operated to place one or more bends in a rod. The French bender requires both handles to operate and provides leverage based on the length of the handle. The use of the French bender requires a high degree of physician skill because the determination of the location, angle, and rotation of bends is often subjective and can be difficult to correlate to a patient's anatomy. Other methods of bending a rod to fit a screw and/or hook construct include the use of an in-situ rod bender and a keyhole bender. However, all of these methods can be subjective, iterative, and are often referred to as an “art.” As such, rod bending and reduction activities can be a time consuming and potentially frustrating step in the finalization of a complex and/or long spinal construct. Increased time in the operating room to achieve optimum bending can be costly to the patient and increase the chance of the morbidity. When rod bending is performed poorly, the rod can preload the construct and increase the chance of failure of the fixation system. The bending and re-bending involved can also promote metal fatigue and the creation of stress risers in the rod.
Efforts directed to computer-aided design or shaping of spinal rods have been largely unsuccessful due to the lack of bending devices as well as lack of understanding of all of the issues involved in bending surgical devices. Recently, in U.S. Pat. No. 7,957,831 to Isaacs, there is described a rod bending system which includes a spatial measurement sub-system with a digitizer to obtain the three dimensional location of surgical implants (screws, hooks, etc.), software to convert the implant locations to a series of bend instructions, and a mechanical rod bender used to execute the bend instructions such that the rod will be bent precisely to custom fit within each of the screws. This is advantageous because it provides quantifiable rod bending steps that are customized to each patient's anatomy enabling surgeons to create custom-fit rods on the first pass, thereby increasing the speed and efficiency of rod bending, particularly in complex cases. This, in turn, reduces the morbidity and cost associated with such procedures. However, a need still exists for improved rod bending systems that allow for curvature and deformity correction in fixation procedures, provide the user with more rod bending options, and accommodate more of the user's clinical preferences.